1. Field of the Invention
This invention relates to a registration system for a web fed die cutting press wherein a continuous, printed indicator line or strip disposed longitudinally on a web is monitored by two sensors for controlling movement of a floating die unit in a direction laterally of the web along what may be called a Y axis and for rotation of the die unit about an upright axis defined as .theta., during the time that the web is advanced in a longitudinal direction along an X axis toward a press work station. Once the presence of an indicia mark corresponding to each area of the web to be processed is detected by a third sensor, advancement of the web is interrupted and a reciprocable ram of the press is shifted to effect immediate die cutting of the associated web area.
2. Description of the Prior Art
Conventional die cut presses known to those in the art are typically sheet fed units with sensors for detecting alignment indicia on each sheet, and wherein the work sheet is shifted as required to bring a defined image area of the sheet into proper registration with work structure of a die unit. While sheet fed presses can generally accurately cut defined individual areas or sections of each sheet, the speed at which such presses process a given number of die cut sheet areas is somewhat limited.
A die cutting press for web material that affords essentially automatic operation while providing a significantly greater speed of press operation in comparison to sheet fed units is disclosed in U.S. Pat. No. 4,555,968, dated Dec. 3, 1985 and assigned to the assignee of the present invention. The press disclosed in U.S. Pat. No. 4,555,968 has a shiftable die unit supported on a cushion of air, and the die unit is movable laterally of the direction of travel of the web as well as rotatably about an upright axis perpendicular to the web in order to bring the die unit into precise registration with defined areas of the web to be processed or cut by the press. Automatic operation of this press is provided by a control system having sensors which detect the presence of the indicia on the web, and the control system is electrically coupled to servomotor mechanisms for adjustably positioning the die unit once advancement of the web is interrupted and a defined area on the web is in general proximity to work structure of the press die unit.
In more detail, the control system of the die cutting press shown in U.S. Pat. No. 4,555,968 has two groups of photo-optical sensors which are disposed to sense the presence of two T-shaped marks that are located on opposite sides of the web adjacent each defined area of the web to be cut. One pair of sensors in each group is oriented to sense opposite sides of laterally extending legs of the respective T-shaped indicia, and each sensor detects reflectance levels that may be defined as -X1, +X1, -X2 and +X2. One of the sensor groups has a second pair of sensors positioned to detect a longitudinally extending leg of the associated T-shaped indicia to provide reflectance level signals that may be termed +Y and -Y. The "X" reflectance levels are analyzed by a microprocessor for positioning the web longitudinally relative to the die unit and to rotatably orient the die unit about an upright axis, or .theta., while the "Y" reflectance levels are received by the microprocessor for positioning the die unit laterally of the web.
During operation of the press shown in U.S. Pat. No. 4,555,968, the web is advanced at a relatively high speed to bring each successive defined image area of the web into proximal relationship with the die unit work structure, and subsequently motors powering web feeding rollers are decelerated and incrementally actuated in a stepping fashion to advance the web in a "creep" mode through successive discrete increments of 0.0005 inch. The "X" reflectance level sensors are monitored by the microprocessor, the die unit is rotated as needed and the web continues to be advanced in the creep mode until the defined web area to be processed is correctly positioned about the .theta. axis as well as along the X axis (i.e. in a direction parallel with the length of the web). Subsequently, advancement of the web is interrupted, the "Y" reflectance levels sensors are monitored by the microprocessor and the die unit is shifted as needed along the Y axis (i.e. laterally of the web length) until the defined web area is in proper registration with the die unit. Next, the microprocessor deactivates air delivery to air bearings floatably supporting the die unit so that the latter comes to rest in a fixed position on a base platen in order that the ram of the press can be shifted to effect die cutting of the defined web area.
Although the web fed die cutting press having an automatic 3-axis die registration system as illustrated in U.S. Pat. No. 4,555,968 represents a significant advance in the art, it would of course be desirable to increase the operational speed of such a press so that greater amounts of web material can be processed. However, it is important that extreme registration accuracy of the die unit with the defined web areas be maintained so that the defined areas are precisely severed from the web within very small dimensional tolerances.